Metal oxides in general are compounds which contain both a metal and oxygen (e.g. MgO, CeO.sub.2, Y.sub.2 O.sub.3, and ZrO.sub.2). Multicomponent metal oxides are those metal oxides which contain two or more different metals. Examples of multicomponent metal oxides include: HgBaCuO, YBaCuO, BiSrCaCuO, TlCaBaCuO, the perovskites ABO.sub.3, where A and B include La, Sr, Al, Ta, Ti, etc. . . . , (i.e. LaAlO.sub.3, SrTiO.sub.3, BaTiO.sub.3, CaZrO.sub.3, and BaZrO.sub.3) and other compounds such as MgAl.sub.2 O.sub.4, SrAlTaO.sub.6, and SrAlNbO.sub.6. Certain multicomponent metal oxides are superconducting and some are superconducting at high temperatures. Metal oxides and multicomponent metal oxides may be used as substrates on which superconducting films are grown.
Superconductivity refers to that state of metals and alloys in which the electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes a transition from a state of normal electrical resistivity to a state of superconductivity is known as the critical temperature (T.sub.c).
Until recently, attaining the T.sub.c of known superconducting materials required the use of liquid helium and expensive cooling equipment. However, in 1986 a superconducting material having a T.sub.c of 30K was announced. See, e.g., Bednorz and Muller, Possible High Tc Superconductivity in the Ba-La-Cu-O System, 64 Z.Phys. B-Condensed Matter 189 (1986). Since that announcement superconducting materials having higher critical temperatures have been discovered. Currently, superconducting materials having critical temperatures in excess of the boiling point of liquid nitrogen, 77K at atmospheric pressure, have been disclosed.
Superconducting compounds consisting of combinations of alkaline earth metals and rare earth metals such as barium and yttrium in conjunction with copper (known as "YBCO superconductors") were found to exhibit superconductivity at temperatures above 77K. See, e.g., Wu, et al., Superconductivity at 93K in a New Mixed-Phase Y-Ba-Cu-O Compound System at Ambient Pressure, 58 Phys. Rev. Lett. 908 (1987). In addition, high temperature superconducting compounds containing bismuth have been disclosed. See, e.g., Maeda, A New High-Tc Oxide Superconductor Without a Rare Earth Element, 37 J. App. Phys. L209 (1988); and Chu, et al., Superconductivity up to 114K in the Bi-Al-Ca-Br-Cu-O Compound System Without Rare Earth Elements, 60 Phys. Rev. Lett. 941 (1988). Furthermore, superconducting compounds containing thallium have been discovered to have critical temperatures ranging from 90K to 123K (the highest critical temperatures to date). See, e.g., Koren, Gupta, and Baseman, 54 Appl. Phys. Lett. 1920 (1989). All of these superconducting compounds are multicomponent metal oxides.
These high temperature superconductors (HTSs) have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, thin films on various substrates have been prepared which have proved to be useful for making practical superconducting devices. The substrate on which a thin film HTS is grown may be a metal oxide. In addition, both the thin film HTS and the substrate on which it is grown may be multicomponent metal oxides.
Several crystal growth techniques have been applied to the growth of high temperature superconducting compounds in thin film form. The most prominent of these have been pulsed laser deposition (PLD), off axis and on axis sputtering techniques, molecular beam epitaxy (MBE), and organometallic vapor phase epitaxy (OMVPE).
Pulsed laser deposition (PLD) was one of the first deposition techniques. See, e.g., Dijkkamp, et al., Preparation of Y-Ba-Cu Oxide Superconductor Thin Films Using Pulsed Laser Evaporation Form High T.sub.c Bulk Material, 51 Appl. Phys. Lett. 619 (1987). In addition, PLD was one of the most successful deposition techniques, particularly for the YBa.sub.2 Cu.sub.3, O.sub.7-.delta. high temperature superconducting compound. See, e.g., Hubler ed., Pulsed Laser Deposition, 17 MRS Bulletin, No. 2, 26 (1992). PLD has been used to produce very high quality films on up to two inch wafers, but users have not been able to eliminate film defects which result from laser beam interaction with target material. In addition, once a target is compounded with a particular composition, the only way to adjust the resulting film composition is to compound a new target.
Off axis and on axis sputtering techniques have also produced very good thin film superconductors on up to two inch wafers. See, e.g., Newman, et al., YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Superconducting Films with Low Microwave Surface Resistance Over Large Areas, 57 Appl. Phys. Lett. 520 (1990); and Poppe, et al., Low-Resistivity Epitaxial YBa.sub.2 Cu.sub.3 O.sub.7 Thin Films With Improved Microstructure and Reduced Microwave Losses, 71 J. Appl. Phys. 5572 (1992). Sputtering is a relatively simple in situ growth technique which may be used to produce films with very few non-deposition related film defects. However, with on axis sputtering, as with PLD, the composition of the growing film can only be changed by compounding a new target. With off axis sputtering, where multiple targets may be available, no consistent active composition control has been demonstrated. Also, both on axis and off axis sputtering suffer from extremely low film deposition rates.
Molecular beam epitaxy (MBE) after several years of being used to produce films of only moderate quality, has recently been used to produce films that are among the best in the world. See, e.g., Humphreys, et al., Optimization of YBa.sub.2 Cu.sub.3 O.sub.7 Thin Films for Multilayers, 27 IEEE Transactions On Magnetics 1357 (1991). Key to these improved results was the adoption of real time composition control of the growing films by utilization of mass spectroscopy. However, as with all MBE deposition processes, the use of high pressures of oxygen gas (HTS materials are all oxides) is difficult, and places severe constraints on the eventual scale up of the process while still producing films of the highest quality.
Organometallic vapor phase epitaxy (OMVPE) is a technique for growing metal oxide and multicomponent metal oxide films. Film growth by this basic technique is known by various names in addition to OMVPE, including chemical vapor deposition (CVD), vapor phase epitaxy (VPE), metalorganic CVD (MOCVD), and plasma enhanced CVD (PECVD), among others. For clarity, OMVPE will be used throughout the present specification.
OMVPE, similar to the case of MBE, has for several years been used to produce thin films of only moderate quality. Early work was on relatively small area wafers deposited at high temperatures. See, e.g., Yamane, et al., Y-Ba-Cu-O Superconducting Films Prepared On SrTiO.sub.3 Substrates By Chemical Vapor Deposition, 53 Appl. Phys. Lett. 1548 (1988). OMVPE has progressed to larger area wafers (three inch) at reduced temperatures. See, e.g., Chern, et al., Epitaxial Thin Films of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. on LaAlO.sub.3 Substrates Deposited by Plasma-Enhanced Metalorganic Chemical Vapor Deposition, 58 Appl. Phys. Lett. 185 (1991). OMVPE further progressed to the use of novel starting material configurations and even larger area wafers (four inch) for deposition. See, e.g., Hiskes, et al., Single Source Metalorganic Chemical Vapor Deposition Of Low Microwave Surface Resistance YBa.sub.2 Cu.sub.3 O.sub.7, 59 Appl. Phys. Lett. 606 (1991). The difficulty of working with metalorganic precursors, in particular their extremely low vapor pressures and their unpredictable decomposition when held at the elevated temperatures necessary for vapor transport, has plagued all researchers to date, and has limited the quality of the films produced. Thus, despite inherent advantages of the OMVPE process, which include compatibility with oxygen at elevated pressures, ability to scale to very large deposition areas, and ability to produce films virtually free of non growth related defects, to date its application to the deposition of HTS materials has been hindered by the poor chemical properties of the currently available precursor compounds.
The basic OMVPE technique includes flowing a gaseous phase of metal containing compounds in the vicinity of a heated substrate wafer whereby the various gas molecules may contact the surface of the wafer. Upon contact with the heated wafer surface, certain gas molecules react with the surface and decompose into constituent metal atoms and non-metal atoms. The metal atoms become incorporated as a solid phase on the surface (i.e. become the growing film). The non-metal atoms remain in a gaseous phase and may be removed. Various modifications of this general process include operating at different gas pressures, using different types of metal containing compounds, and adding non-thermal energy sources to decompose the metal compounds (e.g. decomposition using laser light, radio frequency or direct current plasmas, flash lamps, and chemically excited or reactive molecular species).
Using OMVPE for growing metal oxide and multicomponent metal oxide films provides a variety of benefits over other methods (e.g. laser ablation). OMVPE allows for the growth of such films on large areas and is scalable (i.e. it can be used to coat multiple wafers at the same time). However, major problems plague researchers in this field. One of the biggest problems is reproducibility, especially for multicomponent metal oxide films. For example, HTS films are complex materials the growth of which must be highly controlled to achieve desirable results such as uniformity within the film and reproducibility of the overall film composition.
Previous attempts to control the composition of the gaseous phase during OMVPE have not been successful and the growth and composition of multicomponent metal oxide films through OMVPE have been neither uniform nor reproducible.
As mentioned previously, mass spectroscopy has been used for closed loop composition measurement and feedback in the MBE growth of HTS compounds. Mass spectroscopy is uniquely suited to the high vacuum environment used in the MBE process. In addition, mass spectroscopy has a very large dynamic range for measuring trace quantities of materials, of particular interest when a source vapor is heavily diluted in a carrier gas. However, its application to OMVPE, which typically operates at considerably higher pressures than MBE is complicated by the need to suitably reduce the pressure in the vicinity of the mass spectrometer head for successful operation. A very simple approach to measuring composition in situ and controlling source fluxes in the OMVPE process for deposition of HTS materials has been developed which incorporates quartz crystal deposition rate monitors. See, e.g., Duray, et al., Pulsed Organometallic Beam Epitaxy of Complex Oxide Films, 59 Appl. Phys. Lett. 1503 (1991). While composition feedback control was demonstrated, the memory effects of the quartz crystal monitors after measurement of several different materials has been performed, and their absolute precision do not appear to be suitable for wide spread application. In addition, the very low growth rates used in the above work avoided the problem of quartz crystal overloading during the deposition process. If a quartz crystal change is required during a deposition run due to build up of material, the recalibration necessary would virtually eliminate possibility of consistent composition measurement and control. During the OMVPE deposition of III-V semiconductor compounds, an ultrasonic composition monitoring and feedback technique has been commercially available and successfully used. See, e.g., Butler, et al., Variations in Trimethylindium Partial Pressure Measured by an Ultrasonic Cell on an MOVPE Reactor, 94 J. Cryst. Growth 481 (1989); and DenBaars, et al., 35 GHz fmax InP JFET Grown by MOCVD Using Tertiarybutylphosphine (TBP), 29 Electronics Lett. 372 (1993). This system, while quite successful at the measurement and control of high volatility metalorganic precursor compounds (e.g., trimethyl gallium) at relatively high pressures (P.gtoreq.200 torr), has not been demonstrated for use with the metalorganic compounds required during deposition of the HTS thin films. In particular, it is known that the entire gas delivery system for HTS OMVPE must operate at temperatures above 200.degree. C. and at pressures as low as possible (e.g. 1-100 torr) due to the extreme low volatility of the metalorganic precursor compounds. In all but the most favorable conditions, this would preclude use of the ultrasonic sensors for composition measurement and feedback.
Despite the desirability of the use of OMVPE for growth of high quality metal oxide films, only incomplete success has been achieved to date.